First of all, in the following description, a method for a mobile station to perform scanning on a neighbor base station is explained.
FIG. 1 is a flowchart for a method of scanning a neighbor base station according to a related art.
Referring to FIG. 1, a serving base station connected to a mobile station broadcasts information on a neighbor base station to all mobile stations within its cell.
The information on the neighbor base station is carried on a neighbor advertisement (MOB_NBR-ADV) message. Table 1 shows one example for a configuration of MOB_NBR-ADV message.
TABLE 1SyntaxSizeNotesMOB_NBR-ADV_Message_Format( ){Management Message Type=498 bitsOperator ID24 bits Unique IDassigned tothe operatorN_NEIGHBORS8 bitsFor(j=0; j<N_NEIGHBORS; j++){Neighbor BS-ID48 bits Physical Frequency32 bits Configuration Change Count8 bitsHysteresis threshold8 bitsMAHO report period8 bitsTVL Encoded Neighbor informationVariableTLV specific}}
Referring to Table 1, the number of neighbor base stations, a neighbor base station identifier (BSID), frequency used by a neighbor base station, channel information of a neighbor base station and the like are included in the MOB_NBR-ADV message.
Having received this message, the mobile station temporarily stops receiving the data transmitted by the serving base station and then sends a scan request (MOB_SCN-REQ) message for obtaining a duration for channel quality measurement from the neighbor base station to the serving base station. Table 2 shows one example for a configuration of MOB_SCN-REQ message.
TABLE 2SyntaxSizeNotesMOB_SCN_REQ_Message_Format( ){Management Message Type = 508 bitsScan Duration12 bits Units are framesReserved4 bits}
While the mobile station is measuring a channel quality of the neighbor base station, the serving base station stops transmitting data to the mobile station. Afterwards, the serving base station is then able to transmit data after the scan duration requested by the mobile station. The serving base station sends a scan response (MOB_SCN-RSP) message to the mobile station in response to the MOB_SCN-REQ. Table 3 shows one example for a configuration of MBO_SCN-RSP message.
TABLE 3SyntaxSizeNotesMOB_SCN-RSP_Message_Format( ){Management Message Type = 508 bitsScan Duration12 bits Units are framesStart Frame4 bits}
Referring to Table 3, the base station secures the scan duration requested by the mobile station. Subsequently, the mobile station waits for a time of a start frame and then starts a base station scan.
For the scan duration, the mobile station receives a preamble of the neighbor base station. Before describing the preamble, a frame structure of IEEE 802.16m system is explained as follows.
First of all, a frame structure of a system, which meets the IEEE 802.16m specification, is configured by a unit of super frame (SFH). Each SFH is transmitted by 20 ms periodicity. One SFH consists of 6, 7 or 8 subframes. Various subframe types and sizes are defined for the SFH according to at least one of a system bandwidth, a cyclic prefix (CP) length and the like. In case of type-1, a subframe length is 0.617 ms. A pattern, at which an uplink/downlink (UL/DL) map for data transmission and reception is located, is indicated by the SFH. In this case, the uplink/downlink (UL/DL) map can be received in each subframe time or each second subframe (i.e., one MP for 2 subframes).
In general, a preamble is transmitted by a frame unit from a base station. And, preambles can be classified into a PA preamble and an SA preamble. In this case, the PA preamble is a primary advanced preamble. And, information on a system bandwidth and carrier setting can be included in the PA preamble. Meanwhile, the SA preamble is a secondary advanced preamble. Information on a cell ID, a base station type and the like can be included in the SA preamble. And, the SA preamble is required for obtaining basic synchronization. In this case, the base station type can include such information as a macro base station, a femto base station and the like.
One super frame generally carries one PA preamble and three SA preambles. A most efficient preamble transmission sequence for a scan includes [SA Preamble], [PA Preamble], [SA Preamble] and [SA Preamble] in order.
When a mobile station scans a neighbor base station operating on an inter-frequency different from that of a serving base station, an interruption time occurs in the course of a communication with the serving base station. This is attributed to the fact that the mobile station receives a preamble of the neighbor base station and then temporarily interrupts the communication with the serving base station to match synchronization in downlink (DL). This is explained with reference to FIG. 2 as follows.
FIG. 2 is a diagram for an interruption time that may occur in the course of performing a scan in a general IEEE 802.16e network.
Referring to FIG. 2, a mobile station sends a scan request message to a serving base station to attempt a scan of a neighbor base station in the course of performing data exchange with the serving base station. In response to the scan request message, having received the scan request message, the mobile station performs a scan of the neighbor base station during a scan length/interval (scan duration/interval) by a frame unit from a timing point indicated by start frame (start_frame) information included in the received message.
Total interruption time occurring due to the scan is 10 ms. That is, the interruption time starts when a base station (BS) switching starts with a third frame. And, the interruption time ends before a start of a next frame after the reception of the preamble of the neighbor base station. Hence, total 10 ms interruption time occurs. In this case, the base station (BS) switching time is variable but is assumed as 1 subframe.
In doing so, although the information necessary for the mobile station is one preamble only, 10 ms is consumed to receive it normally. Therefore, a scan procedure for reducing the unnecessarily occurring interruption time is requested. In addition, in case of the general IEEE 802.16e (WiMAX Rel 1.0 or Rel. 1.5), as scan information can be obtained from one preamble transmitted each frame of 5 ms length, even if a scan interval is set for a random frame, it is able to obtain preamble information of the neighbor base station. However, in case of the IEEE 802.16m system, when additional time synchronization is necessary for inter-frequency due to a sync channel divided into a primary sync channel and a secondary sync channel, acquisition of the primary sync channel is requested or acquisition of the secondary sync channel is necessary for a cell ID based scan. Thus, when the hierarchical sync channel structure is taken into consideration, an efficient scheme for a scan interval setting and indication of the scan interval setting is requested.